出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/05/24 21:56:52」(JST)
Hexavalent chromium (chromium(VI), Cr(VI)) refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via hexavalent chromium, specifically the salt sodium dichromate. Approximately 136,000 tonnes (300,000,000 lb) of hexavalent chromium were produced in 1985.[1] Other hexavalent chromium compounds are chromium trioxide and various salts of chromate and dichromate. Hexavalent chromium is used in textile dyes, wood preservation, and as anti-corrosion and conversion coatings and a variety of niche uses. Chromium hexavalent compounds exist in several forms. Industrial uses of hexavalent chromium compounds include chromate pigments in dyes, paints, inks, and plastics; chromates added as anticorrosive agents to paints, primers, and other surface coatings; and chromic acid electroplated onto metal parts to provide a decorative or protective coating. Hexavalent chromium can be formed when performing "hot work" such as welding on stainless steel or melting chromium metal. In these situations the chromium is not originally hexavalent, but the high temperatures involved in the process result in oxidation that converts the chromium to a hexavalent state.(29 CFR OSHA General Industry 1910)
Inhaled hexavalent chromium is recognized as a human carcinogen.[2] Workers in many occupations are exposed to hexavalent chromium. Problematic exposure is known to occur among workers who handle chromate-containing products and those who weld, grind, or braze stainless steel.[2] Within the European Union, the use of hexavalent chromium in electronic equipment is largely prohibited by the Restriction of Hazardous Substances Directive.
Hexavalent chromium compounds are genotoxic carcinogens. Chronic inhalation of hexavalent chromium compounds increases the risk of lung cancer. (The lungs are the most vulnerable, followed by the fine capillaries in kidneys and intestines). Soluble compounds, like chromic acid, are much weaker carcinogens.[3] Chromate-dyed textiles or chromate-tanned leather shoes can cause or exacerbate contact dermatitis. Ingestion of chromium VI can also cause irritation or ulcers in the stomach and intestines.[4] Of 2,345 unsafe products in 2015 listed by the EU Commission for Justice, Consumers and Gender Equality some 64% came from China, and 23% were clothing articles, including leather goods (and shoes) contaminated with hexavalent chromium.
Hexavalent chromium is transported into cells via the sulfate transport mechanisms, taking advantage of the similarity of sulfate and chromate with respect to their structure and charge. Trivalent chromium, which is the more common variety of chromium compounds, is not transported into cells. Inside the cell, Cr(VI) is reduced first to metastable pentavalent chromium (Cr(V)), then to trivalent chromium (Cr(III)). Vitamin C and other reducing agents combine with chromate to give Cr(III) products inside the cell.[3] According to Shi et al., the damage is caused by hydroxyl radicals, produced during reoxidation of pentavalent chromium by hydrogen peroxide molecules present in the cell.[5]
In the U.S., the OSHA PEL for airborne exposures to hexavalent chromium is 5 µg/m3 (0.005 mg/m3).[6][7] The National Institute for Occupational Safety and Health proposed a REL of 0.2 µg/m3 for airborne exposures to hexavalent chromium.[8]
For drinking water, no United States EPA Maximum Contaminant Level (MCL) exists, though the current MCL for total chromium is based on the assumption that all of it is Cr(VI). California has finalized a Public Health Goal of 0.02 parts per billion (ppb or micrograms per liter)[9] and established a MCL of 10 ppb.[10]
Hexavalent chromium was released from the Newcastle Orica explosives plant on the August 8, 2011. Up to 20 workers at the plant were exposed and 70 nearby homes in Stockton.
The town was not notified until three days after the release of the hexavalent chromium and the accident sparked a major public controversy, with Orica criticised for playing down the extent and possible risks of the leak, and NSW state environment minister Robyn Parker and her department attacked for their slow response to the incident. In December 2011, the 8 August leak was the subject of a NSW parliament upper house inquiry, and the NSW Office of Environment and Heritage ordered Orica to undergo an environmental audit, due to be completed by May 2013.[11][12][13]
In 2010, the Environmental Working Group studied the drinking water in 35 American cities. The study was the first nationwide analysis measuring the presence of the chemical in U.S. water systems. The study found measurable hexavalent chromium in the tap water of 31 of the cities sampled, with Norman, Oklahoma, at the top of list; 25 cities had levels that exceeded California's proposed limit of VI and its less toxic forms.[14] There is currently no drinking water limit established by the EPA specifically for chromium VI. The agency began a toxicology study in 2008, following a report by the National Toxicology Program.[15] EPA released a draft scientific assessment in September 2010[16] and expects to begin rulemaking in 2011 or 2012 based on the final assessment.[15][17]
As of 2010, the California Environmental Protection Agency had proposed a goal of 0.2 parts per billion, despite a 2001 state law requiring a standard be set by 2005.[18] A final Public Health Goal of 0.02 ppb was established in July 2011.[9]
Hexavalent chromium was found in drinking water in the southern California town of Hinkley and was brought to popular attention by the involvement of Erin Brockovich. The 0.58 ppm chromium VI in the groundwater in Hinkley exceeded the Maximum Contaminant Level (MCL) of 0.10 ppm for total chromium currently set by the United States Environmental Protection Agency (EPA). The source of contamination was from the evaporating ponds of a PG&E (Pacific Gas and Electric) power generating plant, which were used to dry the precipitate from the cleaning solution for the cooling stacks.[19] It also exceeded the California MCL of 0.05 ppm (as of November 2008).[20] Note that since no MCL exists for chromium VI the total chromium standards apply.
A more recent study found that from 1996 to 2008, 196 cancers were identified among residents of the census tract that includes Hinkley — a slightly lower number than the 224 cancers that would have been expected given its demographic characteristics.[21][22][23] In June 2013 "Mother Jones" published an article regarding work by the Center for Public Integrity that was critical of the study, and some others by the same researcher.[24]
Average Cr(VI) levels in Hinkley were recorded as 1.19 ppb with a peak of 3.09 ppb. The PG&E Topock Compressor Station averaged 7.8ppb and peaked at 31.8ppb.[25] Compare to the California proposed health goal of 0.06 ppb.[26] The same day the study came out, the plume of contaminated water was reported to be spreading.[27] Ongoing cleanup documentation is maintained at California EPA's page.[28]
In June 2009, the groundwater in Midland, Texas (U.S.), was found to be contaminated with chromium. The Midland groundwater reached higher levels of contamination than in Hinkley with 5250 ppb or 5.25 ppm.[29]
In 2007, ASTDR conducted a study of groundwater and soil pollution emanating from Cambridge Plating Company, Inc. in Belmont, Massachusetts (aka Pure Coat North, LLC) concluding that chromium was detected in 25 of the soil samples collected from property at Cambridge Plating Company, Inc., with total concentrations exceeding nine times the reference media evaluation guide (RMEG) value for childhood exposure to chromium VI (200 mg/kg) and three times exceeding the RMEG value for adult exposure to chromium VI(2,000 mg/kg).[30] The incidence of leukemia among females was elevated in Belmont, MA during 1982–1999 (32 diagnoses observed vs. 23.2 expected). Elevations in females were due to four excess cases in each time period (11 diagnoses observed vs. 6.9 expected during 1988–1993; 13 diagnoses observed vs. 8.7 expected during 1994–1999) while elevations among males were based on one to three excess cases. Chromium soil contamination found on property at Cambridge Plating Company, Inc. is in close proximity to Belmont High School.
The Unified Air Pollution Control District reported high airborne levels of chromium(VI) at an elementary school and fire department in Davenport, California. The substance apparently originated from a local Cemex cement plant. The levels of chromium(VI) were eight times the air district's acceptable level at Pacific Elementary School and ten times at the Davenport Fire Department.[31] The levels detected did not exceed EPA limits. However, the air samples taken by the air district from June to August at the elementary school and fire department in Davenport registered measurements of hexavalent chromium that were up to ten times higher than allowed by California environmental standards.[32] The case highlights the previously unrecognized possible release of chromium(VI) from cement-making.[32][33]
In Chicago's first ever testing for the toxic metal contaminant, results show that the city's local drinking water contains levels of hexavalent chromium more than 11 times higher than the health standard set in California in July 2011. The results of the test showed that the water which is sent to over 7 million residents had average levels of 0.23 parts per billion of the toxic metal. California's Office of Environmental Health Hazard Assessment designated the nation's new "public health goal" limit as 0.02 parts per billion. Echoing their counterparts in other cities where the metal has been detected, Chicago officials stress that local tap water is safe and suggest that if a national limit is adopted, it likely would be less stringent than California's goal.
[34] [35]
On January 7, 2011 it was announced that Milwaukee, Wisconsin had tested its water and hexavalent chromium was found to be present. Officials are saying it is in such small quantities that it is nothing to worry about, although this contaminant has been linked to cancer (In the draft human health assessment for chromium-6 that was released in September 2010 by the EPA for independent expert peer review and public comment, the EPA is proposing to classify hexavalent chromium (or chromium-6) as likely to cause cancer in humans when ingested over a lifetime. The EPA will make a final determination by the end of 2011.)[36] Further testing is being conducted and is expected to be completed by the end of the year.[37]
In 2009, a lawsuit was filed against Prime Tanning Corporation of St. Joseph, Missouri, over alleged hexavalent chromium contamination in Cameron, Missouri. A cluster of brain tumors had developed in the town that was above average for the population size of the town. The lawsuit alleges that the tumors were caused by waste hexavalent chromium that had been distributed to local farmers as free fertilizer. Legal proceedings are still ongoing.[38]
In 2008, defense contractor KBR was alleged to have exposed 16 members of the Indiana National Guard, as well as its own workers, to hexavalent chromium at the Qarmat Ali water treatment facility in Iraq in 2003.[39] Later, 433 members of the Oregon National Guard's 162nd Infantry Battalion were informed of possible exposure to hexavalent chromium while escorting KBR contractors.[40] One of the National Guard soldiers, David Moore, died in February 2008. The cause was lung disease at age 42. His death was ruled service-related. His brother believes it was hexavalent chromium.[41] On November 2, 2012, a Portand, Oregon jury found KBR negligent in knowingly exposing twelve National Guard soldiers to hexavalent chromium while working at the Qarmat Ali water treatment facility and awarded damages of $85 million to the plaintiffs.[42]
The chemistry of the groundwater in eastern Central Greece (central Euboea and the Asopos valley) revealed high concentrations of hexavalent chromium in groundwater systems sometimes exceeding the Greek and the EU drinking water maximum acceptable level for total chromium.[43] Hexavalent chromium pollution here is associated with industrial waste.
By using the GFAAS for total chromium, diphenylcarbazide-Cr(VI) complex colorimetric method for hexavalent chromium, and flame-AAS and ICP-MS for other toxic elements, their concentrations were investigated in several groundwater samples. The contamination of water by hexavalent chromium in central Euboea is mainly linked to natural processes, but there are anthropogenic cases.[43]
In the Thebes – Tanagra – Malakasa basin, Eastern Central Greece,[44] which supports many industrial activities, concentrations of chromium (up to 80 μg/L Cr(VI)) and Inofyta (up to 53 μg/L Cr(VI) were found in the urban water supply of Oropos. Cr(VI) concentrations ranging from 5 to 33 μg/L Cr(VI) were found in groundwater that is used for Thiva's water supply. Arsenic concentrations up to 34 μg/L along with Cr(VI) levels up to 40 μg/L were detected in Schimatari's water supply. In the Asopos River, total chromium values were up to 13 μg/L, hexavalent chromium was less than 5 μg/L and other toxic elements were relatively low.[44]
Toxic poultry feed from quicker chromium-based leather tanning (as opposed to traditional slower vegetable tanning) is entering the food supply in Bangladesh, and chicken meat was the most common source of protein in the country. Tanneries in Hazaribagh, an industrial neighbourhood in Dhaka, emit around 21,600 cubic metres of toxic waste each day. The tanneries also generate as much as 100 tonnes per day of scraps - trimmed raw hide, flesh and fat - which are processed into feed by neighbourhood recycling plants and used in chicken and fish farms across the country. Chromium levels ranging from 350 to 4,520 micrograms (0.35 to 4.52 mg) per kilogram were found in different organs of chickens which had been fed the tannery-scraps feed for two months, according to Abul Hossain, a chemistry professor at the University of Dhaka. The study estimated up to 25% of the chickens in Bangladesh contained harmful levels of Cr (VI).[45]
There is sufficient evidence in humans for the carcinogenicity of chromium[VI] compounds as encountered in the chromate production, chromate pigment production and chromium plating industries.
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